(1) Field of the Invention
The present invention relates to a ducted rotor for a rotorcraft, and also to a rotorcraft having such a rotor, and more particularly a tail rotor of a helicopter.
The invention thus lies in the narrow technical field of aircraft rotors.
(2) Description of Related Art
For example, a helicopter generally has a single main rotor mechanically driven by at least one engine, the main rotor serving to provide the helicopter with lift and with propulsion.
That type of helicopter is also provided with a tail rotor that performs an anti-torque function so as to compensate for the yaw torque created by the rotation of the main rotor, the tail rotor exerting thrust transversely.
Furthermore, the tail rotor enables the pilot to control yaw and steering movements of the helicopter by exerting positive or negative transverse thrust.
A non-ducted tail rotor is known for performing this function, which rotor is referred to as a “conventional” tail rotor, for convenience. Conventionally, the non-ducted tail rotor is mounted on one side near a top end of a tail stabilizer or on one end of the tail boom of the helicopter.
Such a non-ducted tail rotor is in widespread use. Nevertheless, it is also possible to implement a ducted tail rotor, e.g. the rotor known under the trademark Fenestron®.
A ducted tail rotor comprises a rotor arranged in a passage formed through the vertical tail fin of a helicopter, the axis of symmetry of the passage being substantially perpendicular to the vertical anteroposterior plane of symmetry of the helicopter.
As a result, the streamlined structure of the vertical fin of the helicopter surrounds said passage and thus the tail rotor, and the wall of the passage itself is also known to the person skilled in the art as a “duct”, which explains why it is referred to as a “ducted tail rotor”. Such a rotor is referred to as a “ducted rotor” for convenience in the description below.
The streamlined structure then protects the tail rotor from impacts against elements external to the aircraft. Likewise, the streamlined structure increases the safety of ground personnel, by preventing such personnel being injured by the tail rotor.
The streamlined structure that surrounds the passage in which the tail rotor is arranged prevents noise from diffusing forwards, downwards, and rearwards relative to the rotorcraft, whereas a non-ducted tail rotor diffuses noise in all directions.
Conventionally, a ducted rotor has a rotor with a hub carrying a plurality of blades that rotate in the passage of the duct.
Each blade may be fastened to the hub, e.g. by a strip that can deform in twisting, sometimes referred to as a “twist strip”. Bearings having elastomer elements are arranged between each blade and the body of the hub. The bearings give the blade freedom to move in turning about the pitch axis in order to change pitch, and also a certain amount of freedom to perform flapping motion and lead-lag motion.
Each blade may also have a collar carrying a pitch lever. The pitch levers of the rotor are then connected to a pitch control disk. Consequently, the aircraft includes control means for controlling the pitch of the blades via the control disk. Conventionally, such control means include pedals.
Each blade may also include at least one balance weight, such as the weights known as “Chinese weights”.
Furthermore, a ducted rotor blade usually presents a shape that is rectangular with a large amount of twist.
It should be recalled that the geometrical twist of a blade may be defined as the angle formed between the axis of the chord of each section of the blade relative to a reference plane of the blade. Sometimes each section of the blade is twisted relative to the pitch variation axis of the blade by an angle that is identified relative to one such reference plane. Under such conditions, the term “twist relationship” is used to designate how said twist angles vary along the span of the blade.
The blades of a ducted rotor present a large amount of twist, with the twist angle separating two distinct sections possibly being about 20 degrees, for example.
Upstream from the blades, the streams of air in the passage present angles of incidence that vary as a function of their span positions along the blade. Thus, the angle of incidence of the air stream relative to a root of the blade is usually different from the angle of incidence of the air stream upstream from the distal end of the blade.
In order to generate a uniform induced speed, a manufacturer twists each blade so as to take these different angles of incidence into account.
Furthermore, the hub is driven in rotation by a power transmission gearbox fastened to the duct, e.g. by downstream support bars of the rotor. These support bars may be streamlined as to constitute stationary vanes redirecting the flow direction of the stream of air downstream from the rotor. Under such circumstances, the assembly comprising these support bars is sometimes referred to as a stator guide vane assembly for the air stream.
It should be observed that the terms “upstream” and “downstream” are defined relative to the flow direction of air through the passage.
It can be understood that the term “ducted rotor” is used below to designate the entire assembly comprising in particular the duct, the rotor as such, the power transmission gearbox, and the guide vanes present downstream from the blades. The ducted rotor thus includes in particular the duct and the element present inside the passage defined by the duct.
Document FR 1 531 536 describes a ducted tail rotor. Each blade comprises an elongate element capable of twisting that is fastened to the hub by means of a bolt. The elongate element may comprise a plurality of thin strips touching one another.
Document FR 2 719 554 describes a ducted anti-torque rotor with floating blades. Each blade is connected to a hub by a footing portion including at least an arm that is twistable about a pitch-change axis. The arm is then housed in a sleeve of the blade root that presents two bearing surfaces in which the sleeve is journaled with radial clearance in both bearings of the hub.
Document EP 1 778 951 describes an anti-torque device for a helicopter. That device has blades of scimitar shape.
Under such conditions, ducted rotors present advantageous functional characteristics.
Nevertheless, the behavior of a ducted tail rotor of a rotorcraft can differ from the behavior of a non-ducted tail rotor. This behavior may be illustrated by a characteristic curve presenting the thrust developed by a tail rotor as a function of the position of means for controlling the pitch of the blades of the tail rotor, e.g. the position of pedals.
The characteristic curve plotting the response of a conventional non-ducted tail rotor to control action is substantially linear.
In contrast, the characteristic curve of a ducted tail rotor presents a relatively flat portion while the control means are lying in an intermediate range requesting little or even no thrust. That intermediate range corresponds to an intermediate stage of flight that occurs between a stage requesting thrust directed in the direction of rotation of the main rotor, and a stage requesting thrust directed in the direction opposite to the direction of rotation of the main rotor.
In other words, during that intermediate flight stage, modifying the position of the control means does not give rise to a thrust modification equivalent to the thrust modification that would be obtained during stationary stages for a control action of the same amplitude.
This feature is known. Under such circumstances, a pilot knows for example that it is appropriate to move the control means through a greater distance in order to obtain a response from a ducted tail rotor during an intermediate stage of flight. That situation can be uncomfortable, but remains acceptable given the advantages of a ducted tail rotor.
Furthermore, the bearings that hold the blades tend to wear relatively quickly.
Document FR 2 271 121 is remote from the technical field of the invention, presenting a device for coupling a blade to a yoke of a mast of a rotary wing.
That device includes one fastener member per blade. Each fastener member is hinged firstly to the yoke, in particular via an elastomer bearing, and is secondly provided with four tabs bolted to a blade. In addition, each fastener member is connected to a tube for controlling the pitch of the blade.
Also known are Documents FR 2 628 062, EP 0 493 303, and DE 102007062490.